Long Term Evolution (LTE) uses orthogonal frequency division multiplexing (OFDM) in the downlink and DFT-spread OFDM in the uplink. FIG. 1 illustrates an example OFDM symbol. The basic LTE downlink physical resource comprises a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
FIG. 2 illustrates an example radio frame. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length 1 ms. FIG. 2 is a schematic diagram of this LTE time-domain structure.
Furthermore, resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in the time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
LTE comprises virtual resource blocks (VRB) and physical resource blocks (PRB). Resources are allocated to a UE is in terms of VRB pairs. Resource allocations may be localized or distributed. For localized resource allocation, a VRB pair is directly mapped to a PRB pair, and thus two consecutive and localized VRB are placed as consecutive PRBs in the frequency domain. For distributed resource allocation, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain, which provides frequency diversity for data channels transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled. For each subframe, a base station transmits control information about which terminals will receive data on which resource blocks in that downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. The number of symbols (i.e., 1, 2, 3 or 4) is known as the Control Format Indicator (CFI). Downlink subframes also include common reference symbols, which are known by the receiver and are used for coherent demodulation, such as demodulation of control information.
FIG. 3 illustrates an example downlink subframe. The illustrated subframe includes three OFDM symbols (CFI=3) for control information. For some LTE releases, the resource assignments described above may also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH).
Wireless systems, such as LTE, may use link adaptation to adapt parameters of a radio link based on current conditions of the radio link. Fast link adaptation to fading channel conditions may enhance system throughput capacity as well as user experience and quality of service. In fast link adaptation, channel conditions are fed back from the receiver to the transmitter. The feedback may comprise, for example, signal to noise ratio (SNR), signal to interference and noise ratio (SINR), received signal level (power or strength), supportable data rates, supportable combination of modulation and coding rates, and/or supportable throughput. The feedback information may apply to an entire frequency band as in W-CDMA or to a specific portion of it as in OFDM systems such as LTE. The term “channel quality indicator” (CQI) may refer to any such feedback information or messages containing such information.
In LTE downlink, CQI messages are fed back from a mobile station to a base station to help the base station determine a radio resource allocation. The feedback information can be used to determine transmission scheduling among multiple receivers, to select suitable transmission schemes (such as the number of transmit antennas to activate), to allocate appropriate amount of bandwidth, and to form supportable modulation and coding rate for the intended receiver. In LTE uplink, a base station can estimate a channel quality from demodulation reference symbols or sounding reference symbols transmitted by a mobile station.
Table 1 indicates a range of CQI report messages for LTE systems. The CQI table supports modulation and coding scheme (MCS) adaptation over wide-band wireless communication channels. The transition points from a lower-order modulation to a higher-order modulation are determined based on link performance evaluation. These specific transition points between different modulations may provide guidelines for optimal system operation.
TABLE 14-bit CQI Table for LTE (Reproduced from Table 7.2.3-1 of 3GPP TS36.213)Coding rate ×Spectral efficiencyCQI indexModulation1024(bits per symbol)0out of range1QPSK780.152QPSK1200.233QPSK1930.384QPSK3080.605QPSK4490.886QPSK6021.18716QAM3781.48816QAM4901.91916QAM6162.471064QAM4662.731164QAM5673.321264QAM6663.901364QAM7724.521464QAM8735.121564QAM9485.55
Based on CQI reports from a mobile station, a base station can choose the best MCS to transmit data on the physical downlink shared channel (PDSCH). The MCS information is conveyed to the selected mobile station in a 5-bit “modulation and coding scheme” field (IMCS) of the downlink control information. The MCS information may be conveyed by an MCS index.
Table 2 indicates an association between an MCS index and a particular MCS and transport block size (TBS) index. In conjunction with the total number of allocated resource blocks, the TBS index further determines the transport block size used in the PDSCH transmission. The last three MCS entries in Table 2 are for hybrid automatic repeat request (HARQ) re-transmissions. For repeat transmissions, the TBS remains the same as the original transmission.
TABLE 2Modulation and transport block size index table for LTE PDSCH(Reproduced from Table 7.1.7.1-1 of 3GPP TS 36.213)MCS IndexModulationTransport block(IMCS)(Qm)size index (ITBS)0QPSK01QPSK12QPSK23QPSK34QPSK45QPSK56QPSK67QPSK78QPSK89QPSK91016QAM91116QAM101216QAM111316QAM121416QAM131516QAM141616QAM151764QAM151864QAM161964QAM172064QAM182164QAM192264QAM202364QAM212464QAM222564QAM232664QAM242764QAM252864QAM2629QPSKreserved3016QAM3164QAM
Specific TBSs for different numbers of allocated radio blocks are listed in 3GPP TS 36.213. These TBSs are designed to achieve spectral efficiencies based on the CQI reports. More specifically, the TBSs are selected to achieve the spectral efficiencies shown in Table 3 when the number of available OFDM symbols for PDSCH is 11.
TABLE 3Spectral efficiency target for LTE with 11 OFDM symbols for PDSCHMCS IndexModulationSpectral efficiency(IMCS)(Qm)(bits per symbol)0QPSK0.231QPSK0.312QPSK0.383QPSK0.494QPSK0.605QPSK0.746QPSK0.887QPSK1.038QPSK1.189QPSK1.331016QAM1.331116QAM1.481216QAM1.701316QAM1.911416QAM2.161516QAM2.411616QAM2.571764QAM2.571864QAM2.731964QAM3.032064QAM3.322164QAM3.612264QAM3.902364QAM4.212464QAM4.522564QAM4.822664QAM5.122764QAM5.332864QAM6.25
LTE uses HARQ with incremental redundancy. Instead of re-transmitting the same portion of a codeword, different redundancy versions are re-transmitted which provides an extra gain over Chase combining.
If terminal complexity and cost were not a factor, a receiver could include a soft buffer large enough to store all the received soft values. When complexity and cost are concerns, however, the soft buffer size in a terminal is generally limited. For high rate transmissions where a transmitter sends large codewords, a UE may not be able store the complete codeword in its limited buffer. Therefore, the eNB and terminal both need to know the soft buffer size. Otherwise, the eNB might transmit coded bits that the UE cannot store, or the UE may not know these are other bits and confuse them with bits it stores.
FIG. 4 illustrates an example codeword. The illustrated example depicts a simplified complete codeword and a number of soft bits that the terminal can store. The complete codeword comprises systematic bits and parity bits. A soft buffer may be sized to store a subset of these bits. If the eNB and the terminal both know the soft buffer size, then the eNB will not transmit coded bits that the terminal cannot store. The eNB knows how many coded bits the terminal stores, and thus the eNB can use those bits for transmissions or re-transmissions.
FIG. 5 illustrates an example circular buffer for transmission and re-transmission of a transport block. The complete circle corresponds to the soft buffer size of a terminal and not to the entire codeword. In a first transmission, depending on the code rate, the eNB transmits some/all of the systematic bits and none/some of the parity bits. In a re-transmission, the starting position is changed and the eNB transmits bits corresponding to another part of the circumference.
In particular LTE releases, each terminal includes up to eight HARQ processes per component carrier and each HARQ process may include up to two sub-processes for supporting dual-codeword MIMO transmissions. The particular releases divide the available soft buffer equally into the configured number of HARQ processes. Each of the divided soft buffers can be used to store soft values of the received codewords. In case of dual-codeword MIMO transmission, the divided soft buffer may be further divided equally to store the soft values of the two received codewords.
FIG. 6 illustrates an example soft buffer divided into eight portions. The illustrated example depicts buffer allocation for a single-codeword transmission mode. Each buffer corresponds to a codeword. Such an allocation may represent an LTE soft buffer allocation for a PDSCH transmission mode other than modes 3, 4, 8, 9 or 10.
FIG. 7 illustrates an example soft buffer divided into sixteen portions. The illustrated example depicts buffer allocation for a dual-codeword transmission mode. Each buffer is half the size of the corresponding buffer in FIG. 6. Such an allocation may represent an LTE soft buffer allocation for PDSCH transmission modes 3, 4, 8, 9 or 10.
3GPP documentation specifies that the soft buffer size assumed by the encoder is calculated as below:
The circular buffer of length Kw=3KΠ for the r-th coded block is generated as follows:wk=vk(0) for k=0, . . . ,KΠ−1wKΠ+2k=vk(1) for k=0, . . . ,KΠ−1wKΠ+2k+1=vk(2) for k=0, . . . ,KΠ−1
Denote the soft buffer size for the transport block by NIR bits and the soft buffer size for the r-th code block by Ncb bits. The size Ncb is obtained as follows:
      N    cb    =      min    ⁡          (                        ⌊                                    N              IR                        C                    ⌋                ,                  K          w                    )      for DL-SCH and PCH transport channels and
Ncb=Kw for UL-SCH and MCH transport channels, where NIR is equal to:
      N    IR    =      ⌊                  N        soft                              K          C                ·                  K          MIMO                ·                  min          ⁡                      (                                          M                                  DL                  ⁢                  _                  ⁢                  HARQ                                            ,                              M                limit                                      )                                ⌋  
where:
If the UE signals ue-Category-v1020, and is configured with transmission mode 9 or transmission mode 10 for the DL cell, Nsoft is the total number of soft channel bits according to the UE category indicated by ue-Category-v1020. Otherwise, Nsoft is the total number of soft channel bits according to the UE category indicated by ue-Category (without suffix).
If Nsoft=35982720,                KC=        
elseif Nsoft=3654144 and the UE is capable of supporting no more than a maximum of two spatial layers for the DL cell,                KC=2        
else                KC=1        
End if.
KMIMO is equal to 2 if the UE is configured to receive PDSCH transmissions based on transmission modes 3, 4, 8, 9 or 10 as defined in the 3GPP documentation, and is equal to 1 otherwise.
MDL_HARQ is the maximum number of downlink HARQ processes as defined in the 3GPP documentation.
Mlimit is a constant equal to 8.
LTE uses hybrid-ARQ (HARQ), where, after receiving downlink data in a subframe, a terminal attempts to decode the data and reports back to the base station whether the decoding was successful (ACK) or not (NACK). After an unsuccessful decoding attempt, the base station may re-transmit the data.
In a subframe where the UE has an uplink grant for PUSCH transmission, the UE may incorporate the HARQ feedback message in the PUSCH. If the UE is not assigned an uplink resource for PUSCH transmission in a subframe, then the UE may use the Physical Uplink Control Channel (PUCCH) to send the HARQ feedback message.
In LTE systems up to release 11, the set of modulation schemes for both downlink and uplink includes Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (16QAM) and 64 Quadrature Amplitude Modulation (64QAM), corresponding to 2, 4 and 6 bits per modulation symbol, respectively. In LTE, for scenarios with high SINR, such as small cell environments with terminals close to the serving eNB, providing a higher data rate with given transmission bandwidth may be accomplished by using higher order modulation that allows for more bits of information to be carried per modulation symbol. For example, with the introduction of 256QAM, 8 bits are transmitted per modulation symbol. This can improve peak data rate maximum by thirty-three percent as shown in FIG. 8.
FIG. 8 illustrates example bit information for particular modulation schemes at particular SNR levels. The illustrated lines on the graph include QPSK line 80, 16QAM line 82, 64QAM line 84 and 256QAM line 86.
As illustrated, 256QAM provides gains when SINR is sufficiently high in certain scenarios. In practice, the performance of 256QAM is sensitive to transmitter EVM (Error vector magnitude) and Rx impairments.
In 3GPP, support for 256QAM may impact the CQI/MCS/TBS table design and UE category handling. A UE category defines a combined uplink and downlink capability. The parameters set by the UE category are defined in subclauses 4.1-4.2 in 3GPP TS 36.306 V11.1.0. In section 4.2 of the above standard, downlink capabilities for different UE categories are described. Particular capabilities include the “Maximum number of DL-SCH transport block bits received within a TTI” and the “Total number of soft channel bits.”
The field “Maximum number of DL-SCH transport block bits received within a TTI” defines how many information bits a UE is capable of receiving per TTI or subframe. A large value corresponds to a higher decoding capability of the UE. The value corresponds to the use of the largest transport block size together with the highest modulation order used.
The field “Maximum number of bits of a DL-SCH transport block received within a TTI” defines the total number of soft channel bits available for HARQ processing. This number does not include the soft channel bits required by the dedicated broadcast HARQ process for the decoding of system information.
The field “Total number of soft channel bits” defines the value for the parameter Nsoft described above. The parameter controls how many soft channel bits a UE can store (i.e., how many received encoded bits it can store). A higher value corresponds to a larger UE soft buffer size. The parameter is also used for transport block encoding at an eNB.
UE categories 6 to 8 were introduced in LTE release 10, while the remaining UE categories were introduced in release 8. To be backwards compatible with UE categories 6 to 8, a UE indicating category 6 or 7 shall also indicate category 4 and a UE indicating category 8 shall also indicate category 5. Table 4 indicates various UE categories and their parameters.
TABLE 4Downlink physical layer parameter values set by the field UE-CategoryMaximumMaximumMaximumnumber ofnumber of DL-number of bits ofsupportedSCH transporta DL-SCHlayers forblock bitstransport blockTotal numberspatialreceived withinreceived within aof soft channelmultiplexing inUE Categorya TTI (Note)TTIbitsDLCategory 110296102962503681Category 2510245102412372482Category 31020487537612372482Category 41507527537618270722Category 529955214977636672004Category 6301504149776 (4 layers)36541442 or 4 75376 (2 layers)Category 7301504149776 (4 layers)36541442 or 4 75376 (2 layers)Category 82998560299856359827208(Note): In carrier aggregation operation, the DL-SCH processing capability can be shared by the UE with that of MCH received from a serving cell. If the total eNB scheduling for DL-SCH and an MCH in one serving cell at a given TTI is larger than the defined processing capability, the prioritization between DL-SCH and MCH is left up to UE implementation.